The present invention relates to zinc oxide based varistors, and in particular to bismuth-free varistors with controllable voltage versus time response.
In general, a metal oxide varistor comprises a zinc oxide (ZnO) based ceramic semiconductor device with a highly nonlinear current-voltage relationship which may be represented by the equation I=(V/C).sup..alpha., where V is the voltage between two points separated by varistor material, I is the current flowing between the points, C is a constant, and .alpha. is a measure of device nonlinearity and is a number greater than 1. If .alpha.=1, the device exhibits ohmic properties. For values of .alpha. greater than 1 (typically 20-50 or higher for ZnO based varistors), the voltage-current characteristics approximate those exhibited by back-to-back connected Zener diodes but with much greater voltage, current, and energy-handling capabilities. Thus, if the voltage applied to the varistor is less than the varistor breakdown voltage, only a small leakage current will flow between the electrodes and the device is essentially an insulator having a resistance of many megohms. However, if the applied voltage is greater than the varistor breakdown voltage, varistor resistance drops to an extremely low value (tenths of an ohm), permitting large currents to flow through the varistor. Under varistor breakdown conditions, the current through the varistor varies greatly for small changes in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limiting or clamping action is enhanced at higher values of .alpha..
Metal oxide varistors are widely employed as surge arresters for protecting electrical equipment from transients on AC power lines created by switching of electrical apparatus or lightning storms. Varistors are also employed as circuit elements in voltage-shaping circuits for providing regularly shaped voltage pulses in response to specific irregularly shaped current pulses. Surge arrester applications require varistors having breakdown voltages slightly greater than the maximum input voltage of the protected system. If a transient is incident such that the total voltage applied to the system rises above varistor breakdown voltage, varistor current increases rapidly along its characteristic current voltage curve, whereupon the varistor acts as a conductive shunt path for the incident transient pulse and the voltage to the system is clamped at a constant value. In voltage-shaping applications, the varistor is required to operate similarly to a Zener diode to instantaneously clamp the voltage at a specified level. In each application, it is generally desirable that system voltage rise no more than a predetermined amount when the system is subjected to a current pulse of specified size. Varistors with high .alpha. (high nonlinearity) are required for such applications.
It has been found, however, that conventional zinc oxide based varistors containing (Bi.sub.2 O.sub.3) as an additive ingredient, when exposed to a high, constant current pulse (resembling some lightning-induced pulses), do not instantaneously clamp voltage at the desired level. In such varistors, voltage has been found to be time dependent, even when the current pulse is time independent. Thus, a varistor subjected to a high, constant current pulse exhibits an initially higher voltage which decreases to a relatively constant value after a few hundred microseconds. The cause for such voltage "overshoot" is not understood, but is known to be unrelated to inductive effects associated with device leads. A voltage overshoot of even a few microseconds is frequently sufficient to destroy sensitive semiconductor devices. Hence, the importance of controlling varistor overshoot voltage is quite apparent.
The present invention provides a bismuth-free ZnO based metal oxide varistor with an essentially flat voltage versus time response. More specifically, the invention provides a varistor with a controllable voltage versus time response. Careful control of varistor composition additive content in accordance with the invention not only reduces voltage overshoot, but allows fabrication of varistors exhibiting a voltage "undershoot". A voltage undershoot characteristic is desirable, for instance, in varistors employed to provide constant voltage output when subjected to particular irregular current pulses.